Changing ISO - What Happens?

Discussion in 'Film and Processing' started by coneected, Jul 31, 2012.

  1. If you lower the ISO of a particular film what type of changes to the image can one expect? Specifically I would be shooting ISO 400 film at ISO 100. Can anyone give me an idea of what to expect?
  2. If you follow the meters settings I believe you will overexpose by two stops.
  3. It depends on the type of film
    reversal ( slide) film is fairly intolerand of changes.
    there is PUSH processing for E-6 films such as ektachome and fujichrome.
    a slite alteration is possible in some situations. like underexposure when light contrast is low.
    Color print C-41 films have a tolerance for overexposure and also real B&W
    films. But C-41 films are developed by a strict ' machine process"
    B&W real B&W are processed in different developers
    and that time temperature and developer type make the real ISO speed
    hasrder to determine. The characteristics of a particular film vary as well.
    Some films are more tolerant and some are lower contrast and others are higher contrast.
    there is no simple answer.
    many years ago there was a 2x safety factor in b&W films example Plus -x was asa 64 and became asa 125 ( similar to iso rating) But some pros and advanced amateurs were already doing this.
    BTW- some say it was a Kodak secret. that they found a way to double the sensitivity and speed of their B&W films
  4. mva


    I have no idea of the answer to the original question, but I am curious too.
    I have often read of people using, for instance, the new Portra 400 at 200.
    I always wonder: what difference would then it make with the Portra 160?
  5. Shooting a film at a HIGHER ISO speed results in underexposure unless you PUSH process the film, basically
    overdeveloping to make up for underexposing. It's not a free lunch though, you lose quality. How much depends on the

    Shooting a film at a LOWER ISO speed results in overexposure unless you PULL process the film (much less common),
    basically underdeveloping to make up for over exposing. It's not a free lunch either.

    People often mildly overexpose or mildly underexpose film because they like the results better in the negatives. This is
    like shooting Velvia at 40 instead of 50 or HIE at 320 instead of 400.
  6. Film manufactures are suppose to rate the sensitivity of their films according to a set of standards and most do so in a lab with precise measuring equipment.
    Production camera shutters, leaf shutters, and light meters are manufactured with a wider tolerance than the lab equipment.
    Camera and leaf shutters are considered good if they operate within 1/3 stop of the marked speed, 1/3 stop fast to 1/3 stop slow or 20% for speeds under 1/100 and 30% for speeds 1/125 and over.
    When a photographer exposes the film at other than box speed they are A. compensating for shutter or light meter error; B. shifting the scene tones to what they like; C. compensating for light levels beyond what the materials can normally handle; D. creating a special effect.
    The standard for pull/push processing of color films is 10% either side of the base time.
    What results one gets will depend on the film in use, the exposure variance, and the processing variance. Careful testing must be done to get accurate and repeatable results. It is best to change one variable at a time to know what change is giving what result.
  7. With negative films, exposing more will open up shadows. If too much more, the highlights will be blown out.
    I must add that metering technique (or the camera's metering algorithms) make a difference too. I used to set my camera to box speed, but my metering technique took into account that deeper parts of the scene were important, so for example I would point the camera down away from bright sky to take a reading, then re-frame to take the shot. The same could be achieved by just lowering the speed setting on the camera, which could be called "pulling."
    With black and white negative film, with more exposure some reduction of development can be useful if not overdone. This results in a lower contrast neg, useful in high contrast scenes.
  8. James G. Dainis

    James G. Dainis Moderator

    With 400 ISO film a good exposure might be at 1/500 sec. You could shoot at 1/125 sec which would overexpose by two stops.


    You could set the ISO to 100 and the camera meter would call for 1/125 sec exposure.

    Either way you are over exposing by two stops. You will get a denser negative that may be harder to print. Zones 1 - 9 would record as Zones 3 - 11.
  9. You do not say what kind of film is involved or why you are considering changing the speed setting.
    Each film has only one ISO number, which is determined using a defined laboratory technique. It is not possible to change the ISO speed in any way. It is however possible to set your exposure meter at any other film speed number you like - this number is not an ISO speed but an ISO speed equivalent called an exposure index (EI).
    In the case of color negative film, very many people find that exposure at half box speed gives better color saturation (technically, because it places more of the image tones on the straight-line portion of the characteristic curve). With b+w film, one famous exposure method called the Zone System calls for each photographer to determine his/her personal EI for a given film based on his/her equipment and personal preferences. In many cases, photographers standardize their b+w exposures on half box speed coupled with a slight reduction in development time. The Zone System then goes on to describe how with b+w film the process of more exposure/less development can be continued in defined steps (n-1, n-2, etc. ) to lengthen the tone scale and how the same principle can be used in reverse (less exposure/more development: n+1, n+2, etc.) to increase contrast and/or compensate for underexposure.
    Color negative film can be treated in the same way but rarely is (or was), except for certain types of film intended for press photography. It is also possible to push/pull (more/less development than standard) color reversal film, quality will be sharply reduced if film is pushed too far but a grain effect may in certain cases be desirable. More frequently photographers use an EI with color reversal film different from the ISO speed, for example a slight increase in exposure with Velvia or more usually a 1/3 to 1/2 stop less than ISO speed to ensure highlight detail is retained (as I used to do with Ektachrome).
    Color negative film is of relatively low contrast and therefore has a relatively high tolerance of overexposure (exposure latitude) - quality suffers with underexposure. In practice color negatives overexposed by 2 stops may deliver results very close to correctly exposed negatives, but grain will be more noticeable with large prints.
  10. 2 stops overexposure (200% more light). Some photographers feel that a small amount of overexposure (1/3 or 1/2 stop) improves the quality of the negative. Usually through trial and error with a specific film and camera.
  11. in a recent thread,someone com plains that Dwaynes does not "push develop" color negative film.
    This film is usually developed by automatic machinery.
    and as said " this is not part of their business plan"
    Slide film has always been available in a more narrow rage of film speeds.
    and again as said in the other thread, "it responds better to pushing"
    besides Color Negative film is readily available in a variety of ISO ratings.
    With B&W films some reduce box speed and others like acufine and diafine increase it.,
    When I wanted to use an old argus a, and all I had was iso 100 film. it had limited settings, at the fastest and smallest, it would work with iso 100-200.
    but I preferred to use it with slower film. I found a time/dilution for iso 50.
    This is something that can't be done with modern color film.
    I DO wonder how and why the Holga shooters all use iso 400 film.
  12. I never push or pull process my film as quality suffers, I might as well buy film of other speed. I do, however, shoot color negative film at ISO lower than box speed. I would down load the characteristic curve from the film manufacturer and make decision as to which ISO I want to use it.
    I found that the ISO rating of most color negative film isn't the one that would yield the best result but rather the highest ISO that would yield acceptable result.

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